The merozoite interacts in a receptor specific manner with the erythrocyte surface and is the stage against which immunity may work to block invasion. Thus, merozoite surface components are of interest for their role in erythrocyte recognition and as antigens for induction of protective immunity. We are identifying P. knowlesi receptors for attachment to monkey and human erythrocytes and P. falciparum proteins that are involved in the junction between merozoites and erythrocytes. We are studying one antigen on the merozoite surface that undergoes antigenic variation to understand the molecular basis for this variation. We are attempting to adapt P. falciparum to subprimates as an animal model for vaccine trials.